The effects of domain replication resulting from interlayer magnetostatic coupling have been investigated in trilayers which consist of two Co/ Pt multilayers with out-of-plane anisotropy separated by a thick Pt layer. For demagnetized films, the strong correlation between the domain configurations of the two multilayers gives rise to an evolution of the equilibrium domain size with the spacer layer thickness. A good correspondence is found between our numerical calculations and the experimental measurements. These results are an extension of Kaplan's theory of domain size in single ferromagnetic films with out-of-plane magnetization, to the case of interacting ferromagnetic films.